Field of the Invention
The invention relates to a circuit configuration for switching an electrical consumer, particularly an inductive consumer. Specifically, the invention relates to a control device for switching high-power electric motors, namely multiphase drives such as rotary or three-phase current motors, for instance for use in automotive assembly. The invention also relates to a method for limiting a time deviation of a switching current of a consumer.
Such drives are cycled at a high frequency of, for instance, 20 kHz given currents of 100 amperes or more and given a conventional supply voltage of 12 or 14 volts. Under those conditions, switching losses dominate total losses of high-power semiconductor switch elements of the control device. Those switching losses depend on the period of the switching process. For that reason, a switching process must progress as quickly as possible. On one hand, the steepness of the voltage edge dU/dt of the switching edges should be and may be relatively high, because capacitive radiation of leads to control electronics for the electrical consumer plays a subordinate role. On the other hand, stray or leakage inductances affect the switching processes, so that an optimally precise limiting of the rate of rise and fall dI/dt of the switching current is indispensable.
German Published, Non-Prosecuted Patent Application DE 43 30 996 A1 discloses a control device for an electrical consumer, particularly an inductive consumer, in which capacitors are connected in parallel between the gate terminal and the source terminal of power MOS transistors in order to be able to set the time deviation dI/dt of the current and the time deviation dU/dt of the voltage substantially independently of one another. That type of control device is schematically represented in the circuit diagram of FIG. 2 and described in detail below. One problem with such a known control device is that a time period until a minimum switch-on resistance is attained is also substantially prolonged and significantly larger drive powers are required. Therefore, the known device is preferably used for load currents under 10 Amperes.
Another problem is that circuit configurations containing capacitors which are connected parallel to gate-source portions of power transistors tend to oscillate.
It is accordingly an object of the invention to provide a circuit configuration for controlling a switching device for switching an electrical consumer and a method for limiting a time deviation of a switching current of a consumer, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which switching losses can be minimized and a time deviation of a switching current dI/dt can be set at a maximum value. The device should also be usable for very large currents on the order of 100 Amperes or more, and should have a simple structure.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for supplying switchable voltage and/or current to an electrical consumer, comprising a switching device having input terminals. A voltage source has at least two output terminals for tapping at least one high and one low supply potential. A driver circuit is connected to the input terminals of the switching device for driving the switching device. At least one lead is connected between the output terminals of the voltage source and the input terminals of the switching device. At least one leakage inductance is distributed to the at least one lead. A measuring and evaluating circuit detects voltage deviations at the input terminals of the driver circuit arising from current deviations in the at least one leakage inductance of the at least one lead and evaluates the current deviations. A feedback circuit controls feedback of the driver circuit to slow switching processes of the switching device whenever an absolute value of a voltage change at the input terminals of the switching device exceeds a defined threshold value.
With the objects of the invention in view, there is also provided a method for limiting a time deviation dI/dt of a switching current of an electrical consumer with such a circuit configuration. The method comprises the following steps: detecting a value of an induction voltage brought about by current deviations dI/dt at the lead between the high supply potential and the switching device; and regulating at least one of the voltage and current supply of the electrical consumer in dependence upon the detected induction voltage for limiting the time-related current deviation dI/dt to a defined maximum value.
Rapid switching processes cause voltage peaks at all inductances in series with the power switches. They are detected by the detector, and the switching current deviation is readjusted accordingly. The circuit thus automatically adapts to its environment, for instance to assembly-related stray or leakage inductances. In addition, the edge steepness dU/dt of the supply voltage of the consumer is not affected. Furthermore, the control circuit according to the invention requires fewer external switch elements than the above-mentioned known control device.
In accordance with another feature of the invention, the switching device includes two field effect transistors that are connected into a half-bridge, the gate contacts of which are each connected to the output of a driver circuit. The feedback circuit preferably controls these driver circuits in such a way that a voltage with a small absolute value which is detected by the detection device brings about a large gate current of the field effect transistors and thus a large deviation in the switching current dI/dt. A voltage with a large absolute value that is detected by the detection device gives rise to a small gate current of the field effect transistors and thus a small deviation in switching current dI/dt. The current control through the driver circuits thus does not affect the edge steepness dU/dt of the voltages at the field effect transistors.
In accordance with a further feature of the invention, the circuit configuration includes a blocking capacitor that is connected parallel to the voltage source of the consumer.
In accordance with an added feature of the invention, the detection device includes a series circuit of a capacitor with an ohmic resistor. That circuit is connected parallel to this voltage source. The capacitor is thus connected to the high supply potential with one terminal. In order to provide an optimal functioning of the circuit, the capacitive time constant CFxc2x7RF of the detection device approximately corresponds to the inductive time constant LE/RE of the blocking capacitor leads.
In accordance with an additional feature of the invention, the detection device detects the amplitude of voltage peaks that arise at the leads of the blocking capacitor or at the supply terminals of the voltage source as the controlled quantity, and feed it to the feedback circuit as the controlled quantity. The circuit thus adapts to its environment automatically, particularly to the series inductance of the blocking capacitor.
In accordance with yet another feature of the invention, there are provided a common detection device and a common feedback circuit for several driver circuits and field effect transistors, since transistors that switch simultaneously are also jointly responsible for a voltage peak, and non-switching transistors are not affected by the intervention of the feedback circuit.
In accordance with a concomitant feature of the invention, the maximum time-related switching current deviation dI/dt can be adjusted to a value that is still tolerable.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for controlling a switching device for switching an electrical consumer and a method for limiting a time deviation of a switching current of a consumer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.